Thin film deposition plays a key role in the fabrication of almost all opto-electronic or photonic devices, regardless of the means by which such films are created, such as, for example, molecular beam epitaxy (MBE), ion beam assisted deposition (IBAD), sputtering, chemical vapor deposition (CVD), or metal organic chemical vapor deposition (MOCVD).
Although the semiconductor industry represents a large market, many industrial and consumer goods also employ thin films. These include anti-reflection and scratch-resistant coatings for optics (including eyewear), the manufacture of photovoltaic cells, thermal control coatings for residential- and office-window materials, and deposition of thermal- and/or wear-resistant coatings on turbine blades, tools, and bearing surfaces. This list is hardly inclusive; the number of consumer products requiring deposition of a thin-film (or application of a thin-film coating) is estimated to be far larger than the volume of high-tech electronic and opto-electronic goods.
Whatever the application, monitoring and precisely controlling film thickness is key to maximizing the yield of high-quality, affordable parts. Accurate, real-time information on structure, quality, and film composition permits adjustment of process parameters to reliably and repeatably deliver films with the desired properties. In situ measurements now employed for certain film deposition configurations include RHEED (reflection high energy electron difraction), TOF (time of flight) ion beam surface analysis, quartz crystal monitors, and optical probe techniques, including ellipsometry and interferometry.
By far the most commonly used technique, and the only one permitting a wide-band thickness measurement, is the quartz oscillator, which performs a very indirect measurement of film thickness. Conversion from the oscillator's frequency shift to weight of the coating, then to its thickness, is prone to numerous errors arising from subtle changes in material properties (strain, density, age of crystal, temperature of sensor). Also, quartz oscillators provide no index of refraction information. Furthermore, to avoid shadowing the workpiece, crystal monitors must necessarily be placed a few inches away from the part to be coated. This results in a difference in deposition rates between the workpiece and monitor, which can fluctuate randomly from run-to-run and lead to unpredictable changes in indicated thickness.
Interferometry, ellipsometry, and other optical-probe techniques have been under development for many years and can provide a large amount of information on as-grown films. These techniques are among the most common diagnostics for post-deposition examination and evaluation of films. However, when employed as real-time diagnostics, apparatus employed in phase- and polarization-sensitive measurements must be reproducibly positioned on or about the reactor to within a fraction of a wavelength of light, not an easy adjustment to make in an industrial setting. All of these processes typically proceed "blindly," without real-time knowledge of the growing layer's characteristics. Reliable, affordable information on the structure, quality, and composition of the growing film would be welcomed and would surely lead to lower cost, swifter development of new devices, and improved quality.
Furthermore, no remote optical diagnostic can escape the need to probe the deposition region through a window; in real-life deposition systems, material is deposited not just on the substrate but also on chamber walls, fixtures, liners, and most particularly on windows. This is no small detail; windows through which interferometric or ellipsometric measurements are made must be coating-free. Although this condition can be met in a (very expensive and very slow-growing) molecular beam epitaxy (MBE) system, even with shields and shutters it is virtually impossible to achieve in the relatively high-throughput CVD reactors and physical deposition systems characteristically employed in production situations. In these systems, the environment is inherently "dirty", and it is extremely difficult to keep windows from becoming coated.
In addition to these difficulties, many important coating processes require very high temperature substrates; the growth rate and ultimate film thickness achievable by these processes can be effectively limited not by deposition conditions, but by the inability of present optical monitoring techniques to function reliably above 800.degree. C.